Boiler plants, particularly chemical recovery boiler plants are conventionally dimensioned and constructed to operate within a certain, designed power range and capacity. This is naturally reasonable in view of optimization of return for investments. Recently, however, it has become increasingly obvious that the basic dimensioning and design of chemical recovery boiler plants for a certain capacity will involve problems. Expansion of a chemical pulp mill or rationalization and improvement of the process in a chemical pulp mill will naturally increase pulp production. Thus a clear need will also arise to increase the capacity, i.e. the efficiency, of the chemical recovery boiler. Consequently, as the chemical recovery boiler is being dimensioned and designed as a compact construction operating in a certain capacity range, an increase in the capacity or the efficiency may be impossible or it may require very large investments and long downtimes.
In the above-described situation, one of the most significant and serious problems is the insufficiency of the heat transfer area of the superheaters, or the superheater arrangement, designed according to the basic dimensioning of the chemical recovery boiler plant with the new, higher heat load. Moreover, the higher heat load increases the flow rate and temperatures of the flue gases in the superheating area and on the heating surfaces after it. As a result of a rise in the temperatures in the superheating area and on the heating surfaces after it, plugging may be caused, thus impairing the availability of the boiler.
Due to an increase in the effect, or the capacity, of the chemical recovery boiler, the following problems may occur in the superheating arrangement:
The open space between the front edge of the superheating arrangement and the front wall of the boiler is so small that it will be impossible to place a new, supplementary superheating of sufficient size in this open space, due to lack of space. PA1 It a new superheater is installed in the open space, it will decrease the heat flow to the existing screen tubes to such an extent that their water circulation will be endangered. This arises up in chemical recovery boilers having a vertical screen tube system in front of the superheater arrangement. PA1 Due to the increased efficiency, or capacity, it will also be necessary to reduce the temperature of flue gases entering the superheater arrangement and the heating surfaces after it so that excessive flue gas temperatures will not increase the plugging tendency in the area of the superheater arrangement and, particularly, in the boiler bank area. However, it is difficult to lower the temperature of flue gases in compact, conventionally designed chemical recovery boiler constructions, because in normal chemical recovery boiler constructions it is difficult to add tube sections for lowering the temperature of the flue gases in an economical way. PA1 The quantity (flow rate) and temperature of the flue gases will rise, wherein the risk of plugging increases in the superheater area as well as in the sections of riser tubes in the boiler bank area and in the economizer area.
It is feasible that a vertical screen tube system could be placed in front of the superheater arrangement, but it involves a risk of damage, because this structure is not sufficiently rigid, and tubes are usually long in this kind of structure. Such a structure can easily vibrate e.g. as a result of soot blowing, wherein the penetration points of the elements are susceptible to damage. Further, falling clods of salt may damage the long elements.